1. Field of the Invention
This invention relates to gas turbines.
More particularly, the invention relates to gas turbines equipped to operate in such a manner as to reduce air pollution.
2. Description of the Prior Art
Ideally, from a combustion or pollution aspect or both, the primary or combustion zone fuel-air ratio should be kept as close as possible to an optimum value, which may be constant over the operating range of a gas turbine. This does not normally happen, since the fixed geometry of the conventional combustor provides a range of primary or combustion zone fuel-air ratios, which can go from over-rich to over-lean.
The constituent emissions from a gas turbine exhaust are formed by diverse processes, dependent on a number of local environmental conditions. The formation of nitric oxides, for example, depend on conditions opposed to those forming carbon monoxide or hydrocarbons. Particulate emissions are dependent on yet another environment. These opposed conditions complicate the designing of a combustor to achieve optimum results.
Attempts to compensate for the extremes of local environment experienced in the cycling of a gas turbine give rise to problems. For example, since nitric oxide formation rate depends essentially on the temperature in the reaction zone and the availability of dissociated or free oxygen, its formation can be related to the residence time of the reactants in the primary or combustion zone, or, to the length of the fluid flow path within the burner, from the fuel nozzle to the dilution zone. An early or accelerated admission of cooling or dilution air can quench the reaction and restrict nitric oxide formation to low levels. This procedure may, however, increase hydrocarbons, smoke and carbon monoxide formation due to incomplete combustion. The residence time of the combustion gases in the primary zone, due to recirculation, may also have a significant effect on nitric oxide emission, due to the high associated temperatures.
In a normal combustor, at full load, carbon monoxide and hydrocarbons are practically non-existent, whereas nitric oxide emission is at its peak. A combustor optimized for full load pollutant emissions would have a leaner than normal primary zone fuel-air ratio, its yield in hydrocarbons and carbon monoxide would be higher, whereas nitric oxides would be considerably reduced. Such a combustor would not be practical for a normal application where the fuel-air ratio is varied over a wide range, especially its stability would be poor and at idle, the hydrocarbons and carbon monoxide emissions would be very high.